Electrolyte composition

ABSTRACT

The electrolyte composition is used in a method of depositing metals, in particular, onto substrates, especially solar cells. The electrolyte composition is particularly suitable for the deposition of metals, in particular silver, onto solar cells. The electrolyte composition is preferably free of cyanides and contains at least one metal, preferably silver, and an iminodisuccinate derivative, preferably a sodium or postassium iminodisuccinate.

CROSS-REFERENCE

The subject matter described and claimed herein below is also describedin German Patent Application No. 10 2009 029 558.5, filed on Sep. 17,2009 in Germany. This German Patent Application provides the basis for aclaim of priority of invention for the invention described and claimedherein below under 35 U.S.C. 119 (a)-(d).

BACKGROUND OF THE INVENTION

1. The Field of the Invention

The present application relates to an electrolyte composition for thedeposition of metals onto substrates. The electrolyte composition atleast comprises the metal ions of the metal which should be depositedwith this composition.

2. The Description of the Related Art

The galvanic deposition of metals from electrolyte solutions is known inthe art. For this purpose the respective metal must be dissolved in theelectrolyte solutions to form its ion and to maintain it in this state.For the latter purpose complexing agents are usually used. An importantcriterion for suitable complexing agents is the sufficiently highstability of the respectively formed complex. Cyanide has often beenused in the prior art as the complexing agent, because it formsextremely stable complexes with numerous metals. Especially silver formsexcellent complexes with cyanide and provides good results in adeposition process of that metal on surfaces. However cyanide has thedisadvantage of being extremely toxic so that its use as a complexingagent is not desired.

Therefore much effort has been made to replace the toxic complexingagent cyanide by other complexing agents. One example of this sort ofcomplexing agent is hydantoin and some of its derivatives. The use ofhydantoin as a complexing agent is discussed for example in EP 1 918 426A2.

A further organic complexing agent, which is known in prior art, issuccinimide as well as its derivatives. This complexing agent isprovided for example in U.S. Pat. No. 4,126,524.

Known electrolyte compositions free of cyanide have one crucialdisadvantage, namely very low deposition rates of only 0.8 to 1 μm perminute. A low deposition rate must be considered in long-term design ofa facility for electrolytic deposition. Not only the acquisition andmaintenance costs of the facility will be greatly increased, but alsothe cost of the electrolyte composition. Therefore because of the highprices of particular metals, such as silver, a high deposition ratebecomes increasingly desirable.

A suitable electrolyte composition should also have the ability to formcomplexes with foreign metal ions, which are not intended to bedeposited onto the respective substrate, and thus to maintain thestability of the electrolyte composition during the whole depositionprocess and also thereafter. It is also desirable that the electrolytecomposition survives fluctuations of the pH occurring during thegalvanization process. Succinimide and hydantoin derivatives do not havethese properties.

It is particularly desirable to provide electrolyte compositions whichare suitable for a deposition of metals, in particular silver, ontosolar cells. On the one hand, the galvanic deposition of silver ontosolar cells is particularly desirable, because the applied layer has aconductivity which is near the theoretical conductivity, but on theother hand it is technically challenging. Usually, solar cells have aback side of aluminium. Therefore a mild alkaline pH is extremelyimportant, because otherwise the aluminium would be dissolved which isnot desired. An ideal pH of 9 to 10 has to be provided during the wholedeposition process which requires special electrolyte compositions.

SUMMARY OF THE INVENTION

It is the object of the invention to provide an electrolyte compositionwhich combines a high deposition rate with a high stability with respectto foreign ions and fluctuations of the pH and which at the same timedoes not have toxicity. In addition, the electrolyte composition shouldbe suitable to be used for deposition of metals, in particular silver,onto solar cells.

The aforesaid object is attained according to the invention by thesubject matter of the appended patent claims.

This object is in particular attained by an electrolyte composition forthe deposition of metals onto a substrate, particularly onto a solarcell, which comprises at least one metal ion and which is characterizedby the presence of an iminodisuccinate derivative as a complexing agent.In the following, the iminodisuccinate derivative of the presentinvention is abbreviated as IDSD.

The IDSD in the electrolyte composition of the present invention ischaracterized by the following chemical formula (I):

In the above formula each of the R1, R2, R5 and R6 groups is selectedfrom the group consisting of O, S and NH; each of the R3, R4, R7 and R8groups is selected from the group consisting of OH, NH₂, SH, O⁻ and S⁻,and when at least one of the R3, R4, R7 and R8 groups is an O⁻ or S⁻ theIDSD is a salt comprising a sodium and/or potassium cation. The IDSD hasthe advantage of excellent complexing properties for numerous metals sothat the composition according to the present invention is well suitedfor the deposition of a large number of metals. At the same time, thesensitivity of the electrolyte composition is decreased, since alsoimpurities in the form of foreign ions can form complexes so that in thesolution no precipitation takes place. In addition, IDSD is not toxicand is biodegradable. The IDSD of the present invention is preferablyused in the form of its alkali metal salts. These are for example thesodium salt or the potassium salt, wherein the sodium salt is apreferred embodiment of the present invention. In particularlypreferable embodiments the IDSD is an iminodisuccinate, in particulartetrasodium iminodisuccinate.

According to the present invention the IDSD can be contained in theelectrolyte composition in a concentration of 50 g/L to 250 g/L,provided that the IDSD is the main complexing agent in the composition.In preferred embodiments the IDSD is the main complexing agent and ispresent in the electrolyte composition in a concentration of 150 g/L to225 g/L.

The term “main complexing agent” in the sense of the present inventionmeans that the ratio of the complexing agent which is designated as themain complexing agent to each other complexing agent in the electrolytecomposition is at least 2:1. Preferably, this ratio is 5:1, furtherpreferably it is 10:1 and most preferably the complexing agentdesignated as the main complexing agent is the only complexing agent inthe electrolyte composition.

In alternative embodiments the IDSD is used in combination with anothercomplexing agent. In these embodiments the IDSD is used in aquantitatively smaller proportion than in cases in which the IDSD is themain complexing agent. In embodiments in which the IDSD is not the maincomplexing agent, but is used in combination with another complexingagent the proportions in which the IDSD is present in the electrolytecomposition of the present invention are from 2 g/L to 40 g/L,preferably from 5 g/L and 20 g/L.

Under consideration of the above-mentioned amounts the IDSD cansubstantially increase the current efficiency of the deposition process.

A further potential complexing agent, which can advantageously be usedin the electrolyte composition, is hydantoin or one or more of itsderivatives. The group of the complexing agents “hydantoin or one ormore of its derivatives” is abbreviated in the following by the genericname, hydantoin. Thus, the scope of the term “hydantoin” in thefollowing encompasses 1-methyl hydantoin, 1,3-dimethyl hydantoin,5,5-dimethyl hydantoin and 1-hydroxymethyl-5,5-dimethyl hydantoin. Inparticularly preferable embodiments besides the IDSD 5,5-dimethylhydantoin is used.

In preferred embodiments the additional complexing agent can be used inthe electrolyte composition in a concentration of 50 g/L to 250 g/L.More preferably the concentration is in a range from 100 g/L to 175 g/L.As already mentioned above, hydantoin cannot form complexes with manyforeign ions. Here the term “foreign ions” means ions of nickel,chromium and iron. However, when hydantoin is used together with IDSD,an electrolyte solution with a very low sensitivity can be prepared.This combination of complexing agents has the additional advantage thathigher amounts of the metal ion can be complexed. For example, thecomplexing agent IDSD alone can only complex up to 20 g/L of silver, butin combination with 5,5-dimethyl hydantoin up to 60 g/L can becomplexed.

The electrolyte composition according to the present invention ispreferably free of sulfonic acid derivatives. Furthermore, theelectrolyte composition according to the present invention is preferablyfree of cyanides.

In preferred embodiments the electrolyte composition according to thepresent invention comprises a conductivity additive. The conductivityadditive improves the conductivity of the electrolyte composition andthus reduces the cell voltage during the galvanization process. This isdesirable, because with a better conductivity of the electrolytesolution an improved current efficiency can be achieved. According tothe present invention, preferably the conductivity additive is acitrate. This citrate is preferably present in a concentration of 20 g/Lto 75 g/L, more preferably 30 g/L to 50 g/L. In this case citrate hasnot only the function of a conductivity additive, but does also formcomplexes with calcium and magnesium ions. In addition, the use ofcitrate has the advantage that the electrolyte solution is not sensitivewith respect to the incorporation of acids, since citrate can act as abuffer when fluctuations of pH occur. Therefore, this addition providesan advantage regarding the stability of the composition. A preferredcitrate for use in the electrolyte composition of the invention istripotassium citrate.

The electrolyte composition according to the present invention ispreferably adjusted to a pH of 8 to 12, more preferably 9.5 to 11. TheIDSD can advantageously be used in the aforesaid pH ranges, since thedegree of protonation of the complexing groups is sufficiently low.

The electrolyte composition according to the present invention issuitable for deposition of different kinds of metals onto numeroussubstrates. Preferably, the electrolyte composition comprises metalions, which are ions of silver, calcium, magnesium, iron, chromium,cobalt, nickel, copper, tin and/or aluminium. In particularly preferableembodiments the electrolyte composition comprises silver ions.

Preferably, the metal ions are present in the electrolyte composition ina concentration of 10 g/L to 60 g/L, preferably of 15 g/L to 40 g/L. Aparticularly preferable embodiment relates to an electrolyte compositionin which IDSD is the only complexing agent and in which the proportionof metal ions is limited to at most 20 g/L.

In preferred embodiments of the present invention the electrolytecomposition comprises a wetting agent in addition to the IDSD. Thewetting agent increases the wettability of the substrate and thusresults in an easier galvanic deposition. It has surprisingly been foundthat the addition of a wetting agent allows the deposition of glossymetal layers onto the substrate. On the contrary, with the methods andelectrolyte compositions of the prior art only lustreless surfaces canbe produced. The wetting agents are surface-active substances comprisinga hydrophilic and a lipophilic part. The hydrophilic part preferablycomprises a polyalkylene oxide chain, in particular a PEG chain. Thewetting agent polyethylene glycol octyl (3-sulphopropyl)diether is mostpreferable.

The use of the electrolyte composition for the galvanic deposition ofmetals onto substrates is a further part of the present invention.Preferably, the substrates are selected from electrically conductivematerial. In particular, glass, metal, metal alloy and/or semiconductorsubstrates are preferable. Glass and silicon substrates are particularlypreferred, but silicon is the most preferred substrate material.

A method for silvering substrates, in particular the above-mentionedsubstrates is also another aspect of the present invention. The methodpreferably comprises the steps of placing the substrate in theelectrolyte composition according to the present invention and applyinga voltage between an anode and a cathode in electrical contact with theelectrolyte composition, wherein the cathode is the substrate.

Preferably, the current density is about 1.5 A/dm².

With an increase of temperature the substrate can be electroplated withan increased current density. Temperatures of 0 to 100° C., particularlyof 20 to 70° C. are preferred.

Preferably, the electrolyte composition is stirred during thegalvanization process, to allow homogenous deposition.

Surprisingly it has been found that the electrolyte compositionsaccording to the present invention are suitable to deposit metals evenwithout current. So in particular embodiments of the method according tothe present invention no voltage is applied. In this case for example itis possible to apply a metal layer onto an electrically conductive glassor a sheet brass. For such a method the temperature preferably has to beat least 40° C.

EXAMPLES

The following specific embodiments of the present invention do not limitthe scope thereof, but are for illustration.

Comparative Composition 1

An electrolyte composition composed of the following components wastested:

Silver as silver methane sulfonate 30 g/L Tripotassium citratemonohydrate 40 g/L Potassium hydroxide 65 g/L 5,5-Dimethyl hydantoin 130g/L 

The pH of the composition was 10.3.

Exemplary Composition 1

An electrolyte composition composed of the following components wastested:

Silver as silver methane sulfonate 30 g/L Tripotassium citratemonohydrate 40 g/L Potassium hydroxide 65 g/L 5,5-Dimethyl hydantoin 130g/L  Tetrasodium iminodisuccinate 10 g/L

The pH of the composition was 10.3.

Test Procedure:

The above-mentioned electrolyte according to example 1 of the inventionand the electrolyte according to the comparative example were filledinto separate beakers, and in each beaker a brass sheet having thedimensions of 10 cm×7 cm was electroplated on one side with non-pulseddirect current for 10 min. Both electrolytes were stirred with arespective stirrer at 400 rpm and the current strength was 0.75 A.Higher current densities could not be achieved with these electrolytesin the beakers.

After the galvanization process on the brass sheet, which has beentreated with the solution according to the present invention, a weightincrease of 498 mg was measured, whereas the weight increase in the caseof the comparative electrolyte composition without IDSD was only 333 mg.

In a further test the coating of solar cells with the electrolytecompositions of the comparative example and the example 2 according tothe present invention was performed. This test was otherwise conductedin the same way as the first test, except that the electrolytecompositions were stirred with respective stirrers at 210 rpm and acurrent strength of 150 mA was used over a time period of 7.5 min.

Exemplary Composition 2

Silver as silver methane sulfonate 30 g/L Tripotassium citratemonohydrate 40 g/L Potassium hydroxide 65 g/L 5,5-Dimethyl hydantoin 130g/L  Tetrasodium iminodisuccinate  5 g/L

The pH of the composition was 10.3.

The following specific examples for the deposition of silver onto asolar cell were conducted with the same current density and over thesame period of time. The solar cell was of the size of 125×125 mm with6% of conductive area.

With the electrolyte composition according to the present invention inthe case of a voltage of 0.7 V a weight increase of 353 mg on the solarcell was measured. The current efficiency was 70.5%. The electroplatedlayer thickness at the contact finger was 11.9 μm.

In the case of a voltage of 0.59 V with the electrolyte solution of thecomparative example a weight increase of 320 mg on the solar cell wasmeasured. The current efficiency was 63%. The electroplated layerthickness at the contact finger was 9 μm.

Thus it has been shown that, when IDSD is included in the electrolytecompositions with otherwise the same conditions, an improvement ofelectrolyte efficiency can be achieved. The electroplated layerthickness correlates with the galvanization time. As shown, for the sameconditions with respect to current density and time according to thepresent invention a layer thickness which is increased by more than 30%could be achieved. In the case of intended mean layer thicknesses ofabout 7 to 8 μm, with the use of the electrolyte composition accordingto the present invention a considerable advantage in time of much morethan 30% can be achieved.

Exemplary Composition 3

An electrolyte composition according to the present invention with IDSDas the only complexing agent comprises the following components:

Tetrasodium iminodisuccinate (Baypure CX100 200 g/L from Lanxess)Tripotassium citrate monohydrate  40 g/L Potassium hydroxide  65 g/LSilver as silver methane sulfonate  18 g/L Ionogenic wetting agent EA15-90 Raschig  7 ml/L Methane sulfonic acid (for pH adjustment to 10.0) 10 ml/L

Exemplary Composition 4

An electrolyte composition according to the present invention with IDSDas the only complexing agent comprises the following components:

Tetrasodium iminodisuccinate (Baypure CX100 200 g/L from Lanxess) Sodiummethane sulfonate  40 g/L Potassium hydroxide  65 g/L Silver as silvermethane sulfonate  18 g/L Ionogenic wetting agent EA 15-90 Raschig  7ml/L Methane sulfonic acid (pH adjusted to 10.0)  10 ml/L

Test for Improving of Conductivity with Citrate Comparative Composition2

Silver as silver methane sulfonate 30 g/L Tripotassium citratemonohydrate  0 g/L Potassium hydroxide 65 g/L 5,5-Dimethyl hydantoin 130g/L 

The pH of the composition was 10.3.

Exemplary Composition 5

Silver as silver methane sulfonate 30 g/L Tripotassium citratemonohydrate 20 g/L Potassium hydroxide 65 g/L 5,5-Dimethyl hydantoin 130g/L 

The pH of the composition was 10.3.

Results

In the case of a process temperature of 40° C., the electrolytes werestirred with a stirrer at 130 rpm and a current density of 0.75 A. Withan electrolyte solution containing citrate it was possible to deposit347 mg of silver onto the sheet brass, whereas in the case of anelectrolyte solution without citrate only 154 mg of silver wasdeposited. The voltage of 2.4 V and 4 V, respectively, is a measure forthe conductivity of the composition.

While the invention has been illustrated and described as embodied in anelectrolyte composition, it is not intended to be limited to the detailsshown, since various modifications and changes may be made withoutdeparting in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist ofthe present invention that others can, by applying current knowledge,readily adapt it for various applications without omitting featuresthat, from the standpoint of prior art, fairly constitute essentialcharacteristics of the generic or specific aspects of this invention.

What is claimed is new and is set forth in the following appendedclaims.

1. An electrolyte composition for deposition of metals onto a substrate,said electrolyte composition comprising: at least one metal ion, and animinodisuccinate derivative.
 2. The electrolyte composition according toclaim 1, wherein the iminodisuccinate derivative comprises a species ofthe formula (I):

wherein R1, R2, R5 and R6 each represent, independently of each other,O, S or NH, and wherein R3, R4, R7 and R8 each represent, independentlyof each other, OH, NH₂, SH, O⁻ or S⁻
 3. The electrolyte compositionaccording to claim 1, wherein the iminodisuccinate derivative is a salt.4. The electrolyte composition according to claim 3, wherein the salt isa sodium salt or a potassium salt.
 5. The electrolyte compositionaccording to claim 1, wherein said at least one metal ion is selectedfrom the group consisting of silver, calcium, magnesium, iron, chromium,cobalt, nickel, copper and aluminium ions.
 6. The electrolytecomposition according to claim 1, wherein said at least one metal ion isa silver ion.
 7. The electrolyte composition according to claim 1,containing from 10 g/L to 60 g/L of said at least one metal ion.
 8. Theelectrolyte composition according to claim 1, wherein theiminodisuccinate derivative is an alkali metal iminodisuccinate.
 9. Theelectrolyte composition according to claim 1, wherein theiminodisuccinate derivative is a main complexing agent therein and ispresent in a concentration of 50 g/L to 250 g/L.
 10. The electrolytecomposition according to claim 9, wherein the concentration of theiminodisuccinate derivative is from 150 g/L to 225 g/L.
 11. Theelectrolyte composition according to claim 1, containing anothercomplexing agent besides the iminodisuccinate derivative and wherein theiminosuccinate derivative is contained therein in a concentration of 2g/L to 40 g/L.
 12. The electrolyte composition according to claim 11,wherein the concentration of the iminodisuccinate derivative is from 5g/L to 20 g/L.
 13. The electrolyte composition according to claim 1,further comprising a hydantoin in addition to the iminodisuccinatederivative, and wherein said hydantoin is contained therein in aconcentration of 50 g/L to 250 g/L.
 14. The electrolyte compositionaccording to claim 13, wherein said concentration of said hydantoin isfrom 100 g/L to 175 g/L.
 15. The electrolyte composition according toclaim 1, further comprising a hydantoin in addition to theiminodisuccinate derivative, and wherein said hydantoin is 5,5-dimethylhydantoin.
 16. The electrolyte composition according to claim 1, whichis free of cyanides.
 17. The electrolyte composition according to claim1, further comprising a citrate.
 18. The electrolyte according to claim17, wherein said citrate is present in a concentration of 20 g/L to 75g/L.
 19. The electrolyte according to claim 17, wherein said citrate ispresent in a concentration of 30 g/L to 50 g/L.
 20. The electrolytecomposition according claim 1, having a pH of between 8 and
 12. 21. Theelectrolyte composition according to claim 20, wherein said pH is 9.5 to11.
 22. A method of depositing metals on a substrate, said methodcomprising galvanically depositing said at least one metal from the saidelectrolyte composition as defined in claim 1 onto the substrate. 23.The method as defined in claim 22, wherein the substrate is a solarcell.
 24. A method of silvering a substrate, said method comprising thesteps of: a) placing the substrate in an electrolyte compositionaccording to claim 1; and b) applying a voltage between an anode and acathode in electrical contact with the electrolyte composition, whereinthe cathode is the substrate.